1. Field of the Invention
This invention relates generally to boilers that have low NOx systems (e.g., low NOx boilers). More particularly, this invention relates to a method for reducing unburned carbon in low NOx systems.
2. Description of the Related Art
Clean air regulations require a reduction in the flue gas emissions of NOx compounds from coal-fired boilers. It is well known that NOx emissions can be reduced through a "deep staging" technique that controls coal particle burnout as it traverses from the point of origin (e.g., the burner) to the final destination (e.g., the stack). Prior art deep staging techniques endeavor to ensure that combustion air is evenly distributed among all of the burners in the boiler.
In the deep staging technique, combustion in the primary combustion zone of a boiler results in a flue gas containing a high percentage of unburned carbon (UBC) in the fly ash particles. Additional air (i.e., overfire air) is subsequently added to the upper furnace to complete the combustion and burn out the un-combusted coal particles.
The reduction in NOx emissions is generally a function of the fuel-to-air ratio during the first combustion step in the primary combustion zone. The lower the stoichiometric ratio, the larger the NOx emission reduction. Thus, there is ongoing emphasis on reducing the amount of oxygen used in the primary combustion zone so that NOx emissions can be reduced until a minimum point is reached at very fuel rich stoichiometry. Thus, there is a desire to achieve substoichiometric conditions in the primary combustion zone so that NOx emissions can be reduced. Unfortunately, this often leads to an increase in the percentage of UBC in the fly ash.
The most common method of measurement of the UBC in the fly ash is the "loss on ignition" or "LOI" test. If the percent LOI exceeds a specific quantity, typically 4-5%, the fly ash must be disposed of in an ecologically safe manner, which can be prohibitively expensive. On the other hand, if the UBC in the fly ash is at a sufficiently low value, then the fly ash can be sold as a concrete admixture, as a soil stabilizer, and as a filler for asphalt and structural materials, such as bricks.
In view of the foregoing, it would be highly desirable to provide a technique to reduce the UBC or LOI flyash byproduct from a coal-fired boiler operated in a low NOx mode. Such a technique would exploit the clean air benefits of a low NOx system, while allowing the flyash to be sold at a profit, instead of being disposed of at a considerable expense.